This invention relates to a vehicle-carried sound reproduction apparatus.
It is considered that, when a sound reproduction apparatus is used to reproduce music or the like, a sound image reproduced is ideally at the height of the eyes of the listener. Therefore, a speaker is usually disposed at the height of the eyes of a listener.
However, in a vehicle-carried sound reproduction apparatus, it is difficult to dispose a speaker at the height of the eyes of the listener who is a passenger such as the driver of the vehicle or a fellow passenger. Thus, a speaker is in most cases disposed at a lower portion {circle around (1)} of a front door or a lower portion {circle around (2)} of a rear door of a vehicle as shown in FIG. 9A. Accordingly, reproduced sound sounds from a low place, and a sound image is positioned below the eyes of the listener.
In order to avoid the disadvantage, an additional speaker having a comparatively small diameter for reproducing sound in a high frequency region is sometimes disposed at a position {circle around (3)} forwardly of a listener as shown in FIG. 9A. However, where an additional speaker is disposed in this manner, sound in a high frequency region and sound in a low frequency region are outputted from different positions. Therefore, the sounds sound separately from each other to the listener.
It is known that sound has a nature that, as the frequency thereof increases, it becomes more likely to be absorbed. Accordingly, where a speaker is disposed at a comparatively low position in a vehicle chamber, sound in a high frequency region is absorbed by a seat or an interior member of the vehicle. Consequently, sound that the listener actually hears is different from reproduction sound outputted from the sound reproduction apparatus.
Further, as a countermeasure against such a situation as described above, it is effective to actually measure a transfer function in a vehicle chamber and correct reproduction sound in accordance with the transfer function. The countermeasure, however, requires a digital signal processing apparatus of a high performance. Since such a digital signal processing apparatus as just mentioned is considerably expensive, it is difficult to use it for sound reproduction apparatus for consumers.
Besides, there usually is a tendency that, if reproduction sound is corrected in accordance with a transfer function, then sound in a high frequency region is emphasized. Thus, when the sound volume level is raised, the sound provides an unfamiliar feeling to the listener.
It is an object of the present invention to provide a vehicle-carried sound reproduction apparatus such as a car audio apparatus which reproduces a sound image in front of a passenger on the vehicle.
In order to attain the object described above, according to an aspect of the present invention, there is provided a vehicle-carried sound reproduction apparatus, comprising: a correction filter circuit for converting input digital audio signals XL(Z) and XR(Z) for left and right channels into digital audio signals YL(Z) and YR(Z) represented, using head transfer functions, respectively by                                                         YL              ⁡                              (                Z                )                                      ·                          xe2x80x83                        ⁢                          GLL              ⁡                              (                Z                )                                              +                                    YR              ⁡                              (                Z                )                                      ·                          GLR              ⁡                              (                Z                )                                                    =                                            XL              ⁡                              (                Z                )                                      ·                          FLL              ⁡                              (                Z                )                                              +                                    XR              ⁡                              (                Z                )                                      ·                          FLR              ⁡                              (                Z                )                                                                                                                YR              ⁡                              (                Z                )                                      ·                          xe2x80x83                        ⁢                          GLL              ⁡                              (                Z                )                                              +                                    YL              ⁡                              (                Z                )                                      ·                          GLR              ⁡                              (                Z                )                                                    =                                            XR              ⁡                              (                Z                )                                      ·                          FLL              ⁡                              (                Z                )                                              +                                    XL              ⁡                              (                Z                )                                      ·                          FLR              ⁡                              (                Z                )                                                        
where, because the head transfer functions are leftwardly and rightwardly symmetrical with each other, the relationships of FLL(Z)=FRR(Z), FLR(Z)=FRL(Z), GLL(Z)=GRR(Z) and GLR(Z)=GRL(Z) are satisfied, and FLL(Z) is a head transfer function from a first speaker for the left channel positioned forwardly of a listener in a vehicle chamber to the left ear of the listener, FRR(Z) is a head transfer function from a first speaker for the right channel positioned forwardly of the listener to the right ear of the listener, FLR(Z) is a head transfer function from the first speaker for the left channel to the right ear of the listener, FRL(Z) is a head transfer function from the first speaker for the right channel to the left ear of the listener, GLL(Z) is a head transfer function from a second speaker for the left channel positioned at a lower position forwardly of the listener to the left ear of the listener, GRR(Z) is a head transfer function from a second speaker for the right channel positioned at a lower position forwardly of the listener to the right ear of the listener, GLR(Z) is a head transfer function from the second speaker for the left channel to the right ear of the listener, and GRL(Z) is a head transfer function from the second speaker for the right channel to the left ear of the listener; and a digital to analog converter circuit for converting the digital audio signals YL(Z) and YR(Z) outputted from the correction filter circuit into analog audio signals; the analog audio signals outputted from the digital to analog converter circuit being supplied to the second speakers for the left and right channels.
With the vehicle-carried sound reproduction apparatus, even if mounting positions of the speakers are limited, a sound image to be formed by the vehicle-carried sound reproduction apparatus can be positioned at the height of the eyes of the listener which is considered to be an ideal position. Accordingly, an imaginary speaker is disposed forwardly of the listener, and a sound field and a sound image are reproduced by the imaginary speaker. Further, the correction filter circuit has a simplified construction, and even where a digital signal processor (DSP) having a comparatively low processing capability is used for the correction filter circuit, an anticipated object can be achieved. Further, only if the transfer functions are measured, optimum correction can be performed with any model of a vehicle having any configuration.
Further, by averaging a plurality of transfer functions, a correction filter circuit which can be effectively used for a plurality of different models of vehicles can be produced. Accordingly, the correction filter circuit can be universally used as a correction filter circuit whose application is not limited to a specific model of a vehicle.
According to another aspect of the present invention, there is provided a vehicle-carried sound reproduction apparatus, comprising a pair of first signal lines along which a pair of digital audio signals for left and right channels are supplied, a pair of second signal lines along which the digital audio signals for the left and right channels are supplied, first and second delay circuits provided in the first signal lines for providing predetermined time delays to the digital audio signals for the left and right channels supplied along the first signal lines, third and fourth delay circuits provided in the second signal lines for providing predetermined time delays to the digital audio signals for the left and right channels supplied along the second signal lines, a pair of high frequency attenuation filters provided for the second signal lines for attenuating high frequency components of the digital audio signals for the left and right channels supplied along the second signal lines, fifth and sixth delay circuits provided for the second signal lines for providing time delays corresponding to a preceding sound effect to the digital audio signals for the left and right channels supplied along the second signal lines, a first digital to analog converter circuit for converting the digital audio signals for the left and right channels outputted from the first signal lines into analog audio signals, a second digital to analog converter circuit for converting the digital audio signals for the left and right channels outputted from the second signal lines into analog audio signals, a first pair of speakers disposed at left and right positions forwardly in a vehicle chamber for receiving the analog audio signals outputted from the first digital to analog converter circuit to reproduce sound, a second pair of speakers disposed at left and right positions rearwardly in the vehicle chamber for receiving the analog audio signals outputted from the second digital to analog converter circuit to reproduce sound, and means for controlling the delay times of the first to fourth delay circuits in response to a positional relationship between the first and second pairs of speakers and a listener.
With the vehicle-carried sound reproduction apparatus, the phases of reproduction sounds outputted from the speakers when they arrive at the listener can be adjusted by means of the first to fourth delay circuits, and accordingly, a sound image can be positioned definitely and a sound characteristic at a seated position of the listener is augmented.
Further, since high frequency components of reproduction sounds to be outputted from the second speakers are attenuated by the high frequency attenuation filters, the position of a sound image as perceived by the listener on a front seat is prevented from being displaced rearwardly, and a sound image can be positioned definitely also in this regard.
Furthermore, since the fifth and sixth delay circuits provide time delays corresponding to a preceding sound effect to the digital audio signals for the left and right channels supplied along the second signal lines, reproduction sounds outputted from the first speakers at the forward positions precede reproduction sounds outputted from the second speakers at the rearward positions relatively by approximately 10 msec to 20 msec. Consequently, the reproduction sounds outputted from the first speakers at the front positions are emphasized, and accordingly, a sound image can be positioned forwardly
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements denoted by like reference symbols.